1. Field of the Invention
The present invention relates to a thermosensitive recording material which utilizes a coloring reaction, for example, between an electron-donating coloring compound and an electron-accepting compound, more particularly to a light permeable thermosensitive recording material which is suitable for use as a film for a video printer, and especially as an image forming sheet capable of producing, like a silver salt film, a high quality black image by X-ray, magnetic resonance imaging (MRI) and computed tomography (CT) for medical diagnostic and consulting purposes using a medical schaukasten.
2. Discussion of Background
It has been a general practice to form images of interior of the body by CT, MRI and X-ray photography on silver salt films for medical diagnoses using a light plate such as a schaukasten.
In recent years, however, disposal of the waste liquid caused by the wet-type image formation process for a silver salt X-ray film has become a serious problem in the medical field. Further, in line with the trend toward the formation of digital image, there is an increasing demand for a dry process using a transparent film capable of easily producing an image therein. The dry processes currently employed in the medical field include: (1) light-exposing and heat-fixing system, (2) thermal transfer system, and (3) thermosensitive recording system.
Leuco-type thermosensitive recording materials in which an image is formed by reaction of a leuco dye with a developer have been generally used as the thermosensitive recording system. Conventionally, leuco-type thermosensitive recording materials have a construction in which a recording layer containing a colorless or light-colored leuco dye and a developer is provided on a paper. The recording material is heated imagewise using a suitable heating device such as a thermal head to form an image thereon. Because the leuco-type thermosensitive recording materials have various merits that complicated processes such as development and image fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low, they are usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder. Images formed by the leuco-type thermosensitive recording materials have been mostly letters.
One thermosensitive recording material for use in the medical field is a reflection type recording material which is opaque or very low in transparency as a whole and which gives an image that can be observed by light reflected thereon. Another thermosensitive recording material for use in the medical field is a light-permeable type recording material whose images are observed by light transmitted therethrough. The latter type of the recording material, which is superior to the former recording material with respect to the image quality, may be used for medical diagnoses using a schaukasten. Additionally, the light-permeable thermosensitive recording material may be used to form a printing master or for use with OHP (overhead projector).
For diagnoses using schaukastens, the images of interior of the body such as internal organs and bones are required to be exact and to have a high contrast and a long life even when stored under high temperature and high humidity conditions for a long time of at least 10 years. Thus, the light-permeable thermosensitive recording materials for use in the medical field requires much higher level of performance as compared with the conventional thermosensitive recording materials.
In particular, the light-permeable thermosensitive recording materials for diagnostic and consulting purposes should meet with following characteristics:
1. The light-permeable thermosensitive recording material should give an image which has high light-shielding property or which can be recognized with high accuracy when irradiated with a backlight disposed therebehind. Thus, the recording material should have high image reproducibility for a wide range of image density and should give an image having a high contrast. Additionally, the produced image should have good stability and preservability even when subjected to high temperature and/or high humidity conditions.
With regard to the image density, an image produced by light-permeable thermosensitive recording materials is desired to have a maximum density of at least 2.5, more preferably at least 3.0, and to be free of fogging. A mere increase of the thickness of the thermosensitive recording layer for the purpose of increasing the image density is undesirable because the background density is also increased, because the transparency is adversely affected and because the thickness tends to be non-uniform so that the dot image reproducibility is adversely affected.
2. To be suited for observation on a schaukasten, the image should have only a single color throughout the low image density region to the high image density region.
In the case of a reflection-type recording material, the use of a single leuco dye can accomplish the above condition. In the case of a light-permeable recording material, however, the light absorption characteristics of the colorant contribute much to the color. Namely, the light absorption spectrum of a thermosensitive recording layer containing a single leuco dye has large undulations, the color tone of the image varies with the image density. To cope with this problem, a plurality of leuco dyes are used in combination to control the color tone. In this case, however, since the different leuco dyes when developed by reaction with a developer form colors having different preservability, the color tone of the developed image gradually changes with time especially in a low image density region. Thus, it is necessary to select suitable leuco dyes having high preservability and stability even when exposed to high temperature and humidity conditions for a long time.
3. The thermosensitive recording material is desired to be highly transparent, since otherwise the incident light will diffuse so that the image recognizability is lowered. With regard to the transparency, the haze of non-image portions (background) is desired to be 70% or less, more preferably 60% or less, most preferably 20-40% which range is comparable to a silver salt film.
Various proposals have been hitherto made on thermosensitive recording materials for use in the medical field. However, they are still unsatisfactory.
For example, JP-A-H04-91983 proposes a transparent thermosensitive recording material having high transparency and capable of forming black images. The preservability of the images produced in the proposed thermosensitive recording material is unsatisfactory especially when stored in under humid conditions. Further, the image in a low density region becomes greenish or reddish.
JP-B-2773539 proposes a transparent thermosensitive recording material using a first leuco dye forming black color upon development in combination with high melting point leuco dyes forming blue and red color, respectively. JP-A-H10-278431 discloses a transparent thermosensitive recording material using two dyes having a difference in melting point therebetween of not higher than 15xc2x0 C. These materials are not satisfactory with respect to formation of single black image, image density and image preservability.
JP-A-H08-156430 proposes a transparent thermosensitive recording material whose developed black images can be seen not only on a schaukasten but also by being held to the sunlight. The preservability of the images on the proposed thermosensitive recording material is unsatisfactory especially when stored in under humid conditions.
It is, therefore, an object of the present invention to provide a light-permeable thermosensitive recording material which can give images having a high density and good preservability under high humidity conditions and which is suitable for use in the medical field.
Another object of the present invention is to provide a light-permeable thermosensitive recording material which can give images of a single color tone throughout in low and high image density portions and which is suitable for use in the medical field.
It is a further object of the present invention to provide a light-permeable thermosensitive recording material which can give images of a single black color throughout in low and high image density portions.
It is yet a further object of the present invention to provide an image forming method using the light-permeable thermosensitive recording material that has accomplished the foregoing objects.
In accordance with one aspect of the present invention there is provided a light-permeable thermosensitive recording material comprising a light permeable support, and a thermosensitive recording layer formed thereon and comprising a leuco dye, a color developer for said leuco dye, and a binder resin, said developer being a compound represented by the following formula (1): 
wherein R1 stands for an alkyl group having 4-16 carbon atoms or an aminoalkyl group having 4-16 carbon atoms.